Sand

As long as I can remember, I have collected interesting bits of rock, looking at their shapes and colours, and wondering what they were. This was fine as long as I had a garden shed of my own to keep them all in. I read some geology as a hobby and began to recognise a few of them, but then age caught up with me and I had to move to a smaller flat and there is simply no room for more rocks. In fact, I had to dispose of many of my old ones. Eventually, it dawned on me that sand is also rock, although made up of rather small pieces, and would not take up too much room, so why not collect that? After all, I was living at the seaside.

Fig. 1. These translucent green grains from South Point, Hawaii, are olivine. The pounding surf erodes a forty-nine-thousandyear- old volcanic cinder cone made of olivine. As the cone erodes, the olivine crystals become beach sand. Very few beaches consist of pure olivine; however, if there is olivine in a sand, that indicates it is a volcanic region. Magnified 250 times.

I really thought I had invented this hobby and was amazed to discover on the Internet that I was an arenologist or possibly an arenophile (from the Latin arena, meaning sand). There is an International Sand Collectors Society in America, which I joined, and now have email friends all over the place, with whom to exchange sands from different areas and an interesting journal to read.

Fig. 2. The beach at Plum Island, Rowley, Massachusetts, USA, gets its pink color from garnets in the sand. As garnet is denser than most other sand grains, it gets left behind as the waves sweep the less dense material farther away. Magnified 60 times.

Members of the US society have different motives for collecting. Some want to collect sands from every country in the world or just from their own region. Others are interested in coloured or unusual sands. I decided that when I acquired a sand, I wanted to know what it was made of and how it got there. This means I have quite a small collection, compared to the hundreds held by some people, but this just reflects the time I spend studying it, trying to identify the grains and researching the geology of the area surrounding the site where it was collected. My first project was to get a sample from every beach around Mounts Bay in Cornwall, where I live, and then work out why they were different. This is ongoing.

Fig. 3. These roundish grains are ooids, grains of calcium carbonate that precipitated from the ocean water along margins of the Great Bahama Bank, one of three areas in the world where ooids are actively being formed. These are from Joulter Cays, located about ten miles north of Andros Island in the Bahamas. The shallow water over the bank results in strong tidal currents; this keeps the ooids in nearconstant motion. Magnified 75 times.

Their differences became obvious when I bought a binocular microscope on eBay (from China) and a whole new world opened up to me, as I could see every sand grain in detail. I realised that, unlike my previous untidy collection of rocks, with sand samples I needed to keep careful records, with drawings and pictures where possible. This meant that I had to learn the language of sand collecting. My notes on early samples are quite inadequate. For example, a friend on holiday in Arabia brought me a sample of desert sand, which I described as “very fine sand which coats larger grains. The larger fragments area mixture of light and much darker grains with white spots in them”. Not very informative.

Fig. 4. These tiny, glassy orange spherules originate from a fire-fountain volcano that erupted over 3.8 billion years ago. Apollo 17 astronauts discovered this “orange soil” on the rim of Shorty Crater in the Taurus Littrow Valley. Magnified 340 times.

Nowadays, I am able to describe much more carefully, and can use the Wentworth scale for size and confirmation that it is sand. A grain is sand if it is more than 1/16 of a millimetre and less than two millimetres; anything larger is a pebble and anything smaller is silt. To measure in millimetres with some accuracy, I have a plastic card from Geo Supplies in Sheffield, which I can use to determine the size of a grain, the percentage of the area of my sample which is covered by a particular type of grain, the variation in sizes in a sample (this is called sorting) and, for shape, a roundness index. It offers size measurements as ‘um phi’. I have no idea what this is, but the numbers enable me to compare grains, so that is what I use.

Fig. 5. Eroded Quartz Crystal. These tiny grains of sand have eroded over hundreds of millions of years and their original crystal shape is not longer seen (magnified 100 times).

In my notes, my latest description of a sample describes sand from the Great Salt Lake in the USA, sent to me by a society member, which I described as consisting “entirely of ooids,which are fairly well sorted, ranging in size from fine (187) to medium (375). They are all the same colour, almost white and every grain is spherical andopaque; some are cemented together”. This description is better, I think, but recognising the chemical composition of these sand grains is not easy – the only information I can find on the Internet is that ooids in general are usually composed of calcium carbonate, but can be composed of phosphate, chert, dolomite or iron minerals. Well, there is no iron as the grains do not react to a magnet and the lake water is said to contain potassium and calcium, and is polluted with mercury. So, although I cannot be sure what they are made of and in what proportion, it might be safe to suggest they are mainly calcium carbonate

Most sand samples contain quartz, feldspar and some black grains. If the source is on or near granite, I can guess that those black grains might be biotite. However, sometimes, the dark grains glitter or are coloured anything from complete black to shades of brown. They can also be translucent or opaque. Then it is more difficult. They could be lava if the sample has come from a volcanic area, or coal – many coal ships unloaded on beaches in Cornwall in the nineteenth century – or slag. There is a very helpful website called webminerals (www.webmineral.com/) where it is possible to enter just one feature, such as colour, and entering ‘black’ brings up a long list of minerals that are usually black. From the detailed descriptions, it is sometimes possible to identify my black or dark sand grains.

The more sand samples I collect, the more varied and marvellous are the grains. Sometimes, a whole beach can be made of shells or coral and the microscope shows many organic forms or fragments. A famous example is the sand from a beach at Taketomi in Japan, where every grain is a star shaped fossil foraminifera. I was able to buy a sample on eBay to admire, but a word of warning – someone else who had done the same thing sent me a sample of his purchase wondering why it was pink. The microscope showed just a few ‘stars’ in a mass of broken pink glass – fraud can be anywhere money is concerned.

What makes sand so interesting is that samples can be uniform, like the Salt Lake example, or with a huge variety of shapes and colours. No one sand ever appears absolutely identical to another. The books by Dr Gary Greenberg (see the references below) have coloured microphotographs, often at huge magnifications, and there is also a website with a video, which show their variety and beauty.

Fig. 6. Tamarindo Sand Grain – A grain of sand from Tamarindo Beach, Costa Rica, is made of chabazite, a glassy cubic mineral.

How the sand sample got to its location is another consideration. I spend some time leaning on the railings of the Promenade at Mounts Bay, watching longshore drift in operation, noticing how waves very rarely arrive at 90 degrees to the beach, but break from east or west, slowly but surely moving the sand further along the beach in the opposite direction. I try to identify the surrounding rocks or cliffs that are being eroded and also look for the nearest stream or river, which is constantly bringing down new supplies.

Wherever possible, my samples come from below the high water mark, as far out as possible, and somewhere near the centre of the beach, so that they do not contain too many bits of sandwich, ice cream cones or cigarette ends. Kind friends who bring me samples from various deserts have been asked to collect well off the main road. Fortunately, most of them have GPS, so recording the location is not a problem. On beaches, especially on the northern Mediterranean coast, samples can turn out to be anything but local, having been delivered by lorry from well inland, to improve the beach for tourists. Other beaches and dunes have been much disturbed, such as that northeast of Hayle, which was the site of a gunpowder factory (which blew up), several copper mines and a sand extraction company, which still removes sand for building. However, taking a sample from as far out as possible reveals many minerals brought down over thousands of years from the tin and copper mining hills of the mainland. A firm is now trying to get permission to mine these seafloor sand deposits to recover a wide range of minerals. The Scientific American book, Sand, by Siever, reveals that the origin of sand deposits is far from a simple matter and is fascinating reading, especially on the evolution of sandstones.

Fig. 7. Six thousand -year old tree stumps revealed at Penzance after a winter storm’. Most of these stumps are now re-covered with sand, until the next big storm.

Wandering around staring at sand, and no doubt looking quite eccentric, can also sometimes reveal interesting things. I discovered an unknown Mesolithic site in south Devon by observing a tiny flint flake stuck to a sand cliff face, tracing its route to the top and finding an area with many worked flints, in the process of being dug up for a flower bed. Another time, an Irish sand dune, half way down, revealed a prehistoric campsite, complete with pottery, animal bones (including horses’ teeth) and flint implements. And, of course, last year, with members of the Royal Geological Society of Cornwall (the second oldest in the world), I visited the beach at Penzance, where a storm had revealed 6,000-year-old tree stumps beautifully preserved in the sand.

Sand is also a great preservative, as Egyptologists have found. In fact, there is no knowing what you may find, in the sand grains themselves or in the dunes behind the beach, along a river, or far out in the desert. It is always worth looking.

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